1. Technical Field
The present invention relates to a riding work vehicle, which includes right and left wheels that are operable as main driving wheels and respectively driven independently for traveling, at least one caster wheel that is operable as a steering wheel whose operational state is selectable between free traveling and forced traveling and also between free steering and forced steering, and a working machine that is operable for work on the ground.
2. Related Art
A work vehicle equipped with a working machine is available for work on the ground, such as a conventionally known lawn mower, farm tractor, or the like. The work vehicle can include right and left wheels that are operable as main driving wheels and respectively driven independently for traveling by a power source (e.g., an electric motor) and a caster wheel that is a freely steerable steering control wheel.
For example, a riding lawnmower vehicle is an automotive work vehicle that is equipped with a lawnmower as a working machine and enables a worker to ride thereon and perform operations for traveling and for ground work. The lawnmower mounted on the riding lawnmower vehicle is, for example, a lawnmower rotary tool.
The riding lawnmower vehicle is an off-road vehicle that usually travels on the ground surface of a garden or the like to operate for lawn mowing work, although the riding lawnmower vehicle is a type of a vehicle. To this end, the riding lawnmower vehicle includes a power source (e.g., an electric motor) mounted thereon for wheel driving and lawnmower driving. The electric motor can receive electric power from a power supply source (e.g., a battery). If the riding lawnmower vehicle is a hybrid-type riding lawnmower vehicle, the battery can receive electric power supplied from an electric generator that is driven by an internal combustion engine for electric power generation.
For example, as discussed in Japanese Patent Application Laid-Open No. 2008-168869, a hybrid type riding lawnmower vehicle includes an engine and an electric generator that can supply electric power to a power source unit. The power source unit can supply electrical energy to an electric rotary machine. The riding lawnmower vehicle further includes a main frame that supports right and left wheels operable as main driving wheels and right and left caster wheels operable as steering control wheels. The right and left wheels are connected to an axle driving electric rotary machine. The caster wheels are connected to a steering wheel driving electric rotary machine. Further, a steering actuator that can function as a motor is connected to respective steering shafts of the right and left caster wheels, to cause the right and left caster wheels to rotate around their steering shafts, respectively. Further, a controller is provided to set arbitrary steering angles for respective caster wheels in a state where the steering actuator is connected to the steering shafts.
Furthermore, as discussed in Japanese Patent Application Laid-Open No. 2008-168871, another riding lawnmower vehicle includes right and left main driving wheels, right and left caster wheels operable as two steering control wheels, and the main driving wheels are driven by an electric motor and the caster wheels are steered by an electric steering motor. The controller includes a switching module that switches the operational state between a forced steering mode and a free steering mode. When the switching module selects the forced steering mode, the electric steering motor forcibly steers the caster wheels. On the other hand, in the free steering mode, the switching module brings the caster wheels into a freely steerable state by stopping power generation for the electric traveling motor or stopping power transmission for steering from the electric steering motor to the caster wheels.
According to the riding lawnmower vehicles discussed in Japanese Patent Application Laid-Open No. 2008-168869 and Japanese Patent Application Laid-Open No. 2008-168871, the main driving wheels are driven by the electric motor serving as the axle driving electric rotary machine and the caster wheels are driven by the electric motor serving as the steering control wheel driving electric rotary machine. Further, the electric motor serving as the steering actuator is provided to rotate respective caster wheels around their steering shafts.
However, according to the vehicle discussed in Japanese Patent Application Laid-Open No. 2008-168869, if the steering control wheel driving electric rotary machine and the steering actuator are in a deactivated state, the steering control wheel driving electric rotary machine and the steering actuator apply a significant amount of load to the caster wheels when the caster wheels are forcibly rotated for traveling or steering by the driving force of the main driving wheels. In this respect, there is some room for improvement in reduction of the torque required to drive the main driving wheels.
For example, permanent magnet equipped motors having permanent magnet equipped rotors may be used for the motors functioning as the steering control wheel driving electric rotary machine and the steering actuator. In this case, a cogging torque may be generated as resistance by the magnetic reaction force of the stator positioned in a confronting relationship with the permanent magnet in a situation where the caster wheels are driven by the force acting from the ground when the main driving wheels are driven in a non-energized state of the motors.
The cogging torque is a component that resists the vehicle when the vehicle is traveling. The cogging torque also resists the caster wheels when the caster wheels are in the freely steerable state. Therefore, the cogging torque is not preferable. Therefore, a relatively large amount of electric power is supplied to the axle driving electric rotary machine dedicated to the main driving wheels to cause the vehicle to travel at an intended speed and in an intended direction. In this respect, there is some room for improvement in energy saving.
Further, brush equipped motors or the like (i.e., motors other than the permanent magnet equipped motors) may be used for the motors functioning as the steering control wheel driving electric rotary machine and the steering actuator. In this case, a significant amount of sliding resistance occurs between the brush and a commutator and therefore the vehicle is subjected to a relatively large amount of resistance in the non-energized state of the motors when the vehicle is in a traveling or freely steering state. Thus, there is some room for improvement in energy saving.
Further, according to the vehicle discussed in Japanese Patent Application Laid-Open No. 2008-168871, as understood from a structure illustrated in FIG. 17 of Japanese Patent Application Laid-Open No. 2008-168871, a spur gear that constitutes a spur gear mechanism is supported, via a one-way clutch, around one end portion of a lower side rotary shaft fixed to the caster wheel. The spur gear is operatively connected to a rotary shaft of an electric motor.
According to this arrangement, if there is the tendency for the rotational speed of the spur gear to become slower than the rotational speed of the caster wheel, the power transmission from the electric motor to the lower side rotary shaft is blocked, thereby preventing the electric motor from obstructing smooth rotation of the caster wheels. Hence, there is a possibility that energy saving can be attained to a certain extent by disconnecting the electric motors (i.e., the steering control wheel driving electric rotary machines) when the electric motors are deactivated.
Further, as discussed in Japanese Patent Application Laid-Open No. 2008-168871, when the free steering mode is selected, electric power supply to the electric steering motor can be stopped or power transmission from the electric steering motor to the caster wheel can be blocked. To this end, it is useful to provide a clutch mechanism in a power transmission path between the electric steering motor and a driving portion of the caster wheel.
However, the above described Japanese Patent Application Laid-Open No. 2008-168871 has no mention of any mechanism for preventing the steering motor from obstructing the steering operation of the caster wheel that is driven by the force acting from the ground when the vehicle travels by the driving force of the main driving wheels, in a situation where the caster wheel is freely steerable in the deactivated state of the steering motor. Namely, there is no discussion that suggests the clutch that is capable of blocking transmission of the rotational force in the deactivated state of the electric steering motor.
Therefore, in the configuration discussed in Japanese Patent Application Laid-Open No. 2008-168871, electric power is wastefully consumed if the clutch is disengaged to block the power transmission for the steering operation in the power-supply state of the electric steering motor. There is some room for improvement in energy saving.